The present invention relates to a compressor assembly, in particular to a compressor assembly comprising a compressor having a rotatable impeller and a motor driving the compressor, the impeller and the motor being linked by a direct drive.
Compressors having an impeller rotatable within a compressor casing are well known in the art. Such compressors include both centrifugal compressors or radial flow compressors and axial flow compressors. In centrifugal or radial flow compressors, the fluid being compressed is caused by the rotating impeller to flow along a passageway in which the cross sectional area normal to the flow gradually decreases in the direction of flow. Axial compressors operate by causing the fluid to be compressed to flow along a passage of constant or substantially constant cross sectional area. An example of such a compressor is disclosed in U.S. Pat. No. 4,428,715.
Compressors of the aforementioned types may be driven by a range of motors, such as internal combustion engines, and turbines. However, in many applications it is both preferable and desirable to drive centrifugal and axial flow compressors using electric motors. Typically, induction or synchronous electric motors have been employed to drive compressors. To date, a major drawback associated with the use of electric motors to drive rotating impeller compressors has been the linkage between the electric motor and the compressor impeller. A given compressor will have a specific speed of rotation of the impeller in order to achieve the compression duty required of it. At the same time, an induction electric motor will have an optimum speed of rotation, at which the torque output is at a maximum. Heretofore, in order to link the compressor with a suitable electric drive motor, it has been necessary to employ an arrangement of one or more gears. In this way the different optimum speeds of rotation of the compressor and the electric motor can be accommodated. A particular problem arises in the case of high speed centrifugal compressors, having power requirements of the order of 200 horsepower or less. Such compressors are often required to operate at high speeds, which can be in excess of 50,000 rpm. The optimum speed of rotation of an induction electric motor suitable for this duty is far lower than the speed of rotation required of the high speed compressor, requiring a gear assembly to be employed in the drive assembly of the compressor. However, for such compressors, the high costs of incorporating an arrangement of gears in the drive assembly results in a significant economical disadvantage. This in turn has led to other forms of compressors, such as screw compressors, being favored for such duties.
Accordingly, there is a need for a compressor assembly in which the requirement for a gear assembly in the drive is dispensed with and in which the compressor and the electric motor are directly linked. There is an especial need for a direct drive compressor and electric motor assembly capable of operating at the high speeds of rotation specified above.
A rotordynamic machine is disclosed in U.S. Pat. No. 6,043,580, for use in the pumping of a fluid. The machine comprises an electrical assembly in combination with a turbomachine or centrifugal pump. The electrical assembly acts as a combined electric motor and bearing assembly, having a rotor supported and rotated by magnetic fields generated in a stator. In this way, the motor is bearing-free. The rotor of the motor is formed as part of the shaft connecting the electrical assembly with the turbomachine or pump. U.S. Pat. No. 6,043,580 discloses that the combined electric motor and bearing assembly may be arranged on the principles of an induction motor, an asynchronous motor, a reluctance motor, or a synchronous motor. Specific designs mentioned in U.S. Pat. No. 6,043,580 include assemblies having a rotor with one or more permanent magnets and a rotor designed as a cage rotor with a short circuited cage. In the specific embodiment disclosed and described in detail in U.S. Pat. No. 6,043,580, the combined motor and bearing assembly comprises a stator having two sets of current windings, one set for generating the magnetic fields to rotate the rotor, the second set for generating the magnetic journaling for supporting the rotor shaft in position. The rotor is designed as a cage rotor, having the same number of poles as the stator windings generating the drive, but a different number of poles to the stator windings providing the support for the shaft.
U.S. Pat. No. 6,043,580 is concerned specifically with overcoming the problems associated with magnetic bearings and their limited bearing capacity. The solution proposed, as discussed above, is to arrange an electric motor, which may be one of a wide variety of arrangements of electric motor, such that the rotor is both supported and rotated by a magnetic field. U.S. Pat. No. 6,043,580 does not disclose or suggest an assembly for use at the high speeds of rotation specified above.
U.S. Pat. No. 6,056,518 discloses a fluid pump for use in the coolant system for an automobile. The fluid pump disclosed comprises a switched reluctance electric motor, in which the impellor of the pump is the rotor of the electric motor. The operating speeds for the fluid pump disclosed in U.S. Pat. No. 6,056,518 are low, being stated to be from 0 to 5000 rpm. U.S. Pat. No. 6,056,518 specifically teaches that the advantage of using the switched reluctance motor is that it does not rely for operation upon the use of magnets, which are stated to be heavy, costly and to degrade quickly over time.
According to the present invention there is provided a compressor assembly comprising
a compressor having a compressor casing comprising a fluid inlet and a fluid outlet;
an impeller rotatable within the compressor casing;
an electric motor;
a rotatable drive shaft assembly extending from the electric motor into the compressor casing;
the impeller being mounted on the drive shaft assembly and rotatable therewith within the compressor casing; and
the electric motor comprising a stator and a rotor, the rotor being mounted on the drive shaft assembly and rotatable therewith; wherein the compressor assembly operates at a speed of 25,000 rpm or higher.
A range of electric motors may be employed in the compressor assembly of the present invention. Such motors include induction motors, synchronous motors and asynchronous motors.
Surprisingly, contrary to the suggestions in the prior art, it has been found that the use of a permanent magnet electric motor allows a direct drive compressor assembly to be constructed which is particularly suitable for operation at high speeds. Accordingly, a permanent magnet motor is the preferred motor for use in the assembly of the present invention.
It has been found that a permanent magnet motor may be employed to drive a rotating impeller compressor using a direct drive configuration, that is one in which the impeller of the compressor and the rotor of the motor are directly connected and rotate at the same speed. It has been found that a permanent magnet motor may be used to drive the rotatable impeller of a compressor, allowing the gear assembly or gear box to be dispensed with and a direct drive assembly to be employed.
The compressor assembly of the present invention is operated at high speeds. In this respect, high speed operation is considered to be when the compressor and motor operate at speeds of 25,000 rpm and higher. The compressor assembly of the present invention may be operated at speeds of 50,000 rpm, with speeds of 75,000 rpm and higher being possible. With such high speeds of operation, it has been found that the efficiency of the motor design plays an important role. Induction motors, require a magnetic field to be induced in the rotor, which is typically comprised of a plurality of iron laminations. At the high speeds of rotation required of the compressor assembly of the present invention, the need to induce a magnetic field in the rotor leads to a marked inefficiency in the power usage of the motor, in turn leading to an efficient operation of the compressor. It has been found that a permanent magnet motor overcomes these problems of low efficiency encountered with induction motors. Accordingly, while induction motors may be employed in the compressor assembly of the present invention, it is preferred to employ a more efficient motor arrangement, such as a permanent magnet motor, when operating at speeds in the upper regions of the ranges mentioned above.
Further, permanent magnet electric motors are quieter in operation than other forms of motor, in particular switched reluctance motors, and allow a compact motor and compressor assembly to be constructed.
The compressor used in the assembly of the present invention may be either an axial flow compressor, or a centrifugal or radial flow compressor. The preferred embodiment of the present invention employs a centrifugal or radial flow compressor.
Although any size or rating of compressor may be used, the compressor assembly of the present invention offers particular advantages when the compressor has a power input requirement of less than 200 horse power. It has been found that the compressor assembly of the present invention offers significant advantages when the compressor has a power input requirement of from 75 to 200 horse power. The permanent magnet motor is of particular advantage when the power requirement is from 100 to 200 horse power, especially from 100 to 150 horse power.
In its simplest form, the compressor assembly of the present invention comprises an electric motor having a rotor mounted on a shaft, the shaft in turn being connected directly to the impellor of the compressor. Such a compressor assembly thus consists essentially of an electric motor and a single compressor unit.
The compressor assembly preferably comprises first and second compressors having first and second compressor casings, each of the first and second compressor casings comprising a fluid inlet and a fluid outlet. First and second impellers are located within and rotatable within the first and second compressor casings respectively. The first and second impellers are mounted on the drive shaft assembly and are rotatable therewith. Such a compressor assembly may comprise two separate compressors driven from the same permanent magnet motor. More preferably, however, the two compressors are combined to form a two-stage compressor assembly. In such an arrangement, the fluid outlet of the first compressor casing communicates with the fluid inlet of the second compressor casing. In a two compressor assembly or two-stage compressor assembly, the electric motor is most conveniently disposed between the first and second compressor casings, with the rotor of the electric motor being mounted on the drive shaft assembly between the first and second impellers.
References in this specification to a xe2x80x9cdrive shaft assemblyxe2x80x9d are to a linkage transferring drive from the electric motor to the impellers of the compressor assembly. The drive shaft assembly provides a direct drive between the rotor of the electric motor and the impellers. Such a drive is characterized by the motor and the impeller rotating at the same speed. The drive shaft assembly may comprise one or more individual shafts linked by couplings so as to allow the drive to be transferred. A most convenient and advantageous assembly is one in which the rotor of the electric motor and the impeller are mounted on a single shaft.
A preferred embodiment of the present invention is a two stage centrifugal compressor assembly comprising:
a first compressor casing having a fluid inlet and a fluid outlet;
a first impeller rotatable within the first compressor casing;
a second compressor casing having a fluid inlet and a fluid outlet;
a second impeller rotatable within the second compressor casing; and
a permanent magnet motor disposed between the first and second compressor casings and comprising a stator and a rotor rotatable within the stator;
a drive shaft; wherein
the first impeller, second impeller and the rotor are mounted on the drive shaft and rotatable therewith; and
the fluid outlet of the first compressor casing communicates with the fluid inlet of the second compressor casing;
wherein the compressor assembly operates at a speed of 25,000 rpm or higher.